不定式和动名词都可以作表语，一般情况下可以互换。

此外，不定式和动名词作表语时，可与主语互换位置，而意思保持不变。

His job is to raise pigs. = To raise pigs is his job.

Her job is teaching. = Teaching is her job.

一、不定式作表语，往往表示主语的具体内容，具体事件、主语的目的等。从时间概念上说，有的不定式可能表示将来的动作。

    His job is to teach English. 他的工作是教英语。

    The main thing is to keep our machine from rust.主要的事情是使我们的机器不生锈。

    Her only wish is to have a job in an office.她唯一的愿望就是找一份办公室的工作。

    What impresses me most is to see her always busy.给我留下最深印象的是发现她总是很忙。

二、动名词作表语多表示笼统的、抽象的概念。

The boy's hobby is surfing online, which worries his parents a lot.

这个男孩的爱好就是网上冲浪，这让他父母十分担心。

My favourite sport is playing tennis.我最喜欢的运动是打网球。

Her job is teaching（=Teaching is her job）.她的工作是教书。

动名词作表语和进行时态的区别：

Her job is looking after children. (动名词作表语)

“她的工作是照料孩子。”

本句中，因为is是系动词，主语和表语可以互换而意思保持不变：

Looking after children is her job.

She is looking after children.( is和动词现在分词构成进行时态)

“她正在照料孩子。”

本句中，因为is不是系动词，而是跟动词-ing构成进行时态，所以is前后两部分不

互换。不可以说：Looking after children is she.

三、关于这个句子The trick is to keep your body still and your arms relaxed. 可以变成：The trick is keeping your body still and your arms relaxed. 

又如：

The best way is to take things one step at a time.

最好的方法就是一步一步地参与这些活动。

The best way is to prevent disease.

预防疾病才是上策。

The best way is crushing the tablets in milk or orange juice and drinking that. 

最好的方法是将药片弄碎放在牛奶或橘子汁中服用。

If coming across professional beggers, the best way is leaving at once. 

遇到要钱的职业乞丐，最该做的就是直接走开。